// 算平均数

#include <stdio.h>

int main(int argc, char const *argv[])
{
    /* 给定个数求平均
    int count, num, sum = 0;

    scanf("%d", &count);

    for (int t = count; t > 0; t--)
    {
        scanf("%d", &num);
        sum += num;
    }

    printf("%d\n", sum / count);
    */

    // 任意个数，-1结束
    int count = 0, num, sum = 0;

    /* do
    {
        scanf("%d", &num);
        if (num != -1)
        {
            count++;
            sum += num;
        }
    } while (num != -1); */
    scanf("%d", &num);
    while (num != -1)
    {
        count++;
        sum += num;
        scanf("%d", &num);
    }

    printf("%f\n", 1.0 * sum / count);

    return 0;
}